One method commonly used for inflating air bags in vehicle passive restraint systems involves the use of an ignitable gas generating composition that generates an inflating gas due to an exothermic reaction occurring upon deflagration of the composition. Because of the nature of passive restraint systems, the gas must be generated, and the air bag deployed in a matter of milliseconds. For example, under representative conditions, only about 60 milliseconds elapse between primary and secondary collisions in a motor vehicle accident, i.e., between the collision of the vehicle with another object and the collision of the driver or passenger with either the air bag or a portion of the vehicle interior.
In addition, the inflation gas must meet several stringent requirements. The gas must be non-toxic, non-noxious, must have a generation temperature that is low enough to avoid burning the passenger and the air bag, and it must be chemically inert so that it is not detrimental to the mechanical strength or integrity of the bag.
The stability and reliability of gas generating compositions over the life of the vehicle are also extremely important. The gas generating composition must be stable over a wide range of temperature and humidity conditions, and must be resistant to shock, so that it is virtually impossible for the gas generating composition to be set off except when the passive restraint system is activated in the event of a collision.
Typically, the inflation gas is nitrogen, which is produced by the decomposition reaction of a gas generating composition containing a metal azide. One such gas generating composition is disclosed in Reissued U.S. Pat. No. Re. 32,584. The solid reactants of the composition include an alkali metal azide and a metal oxide, and are formulated to ignite at an ignition temperature of over about 315.degree. C.
The gas generating composition is typically stored in a metal inflator unit mounted in the steering wheel or dashboard of the vehicle. Several representative inflator units are disclosed in U.S. Pat. Nos. 4,923,212, 4,907,819, and 4,865,635. The combustion of the gas generating composition in these devices is typically initiated by an electrically activated initiating squib, which contains a small charge of an electrically ignitable material, and is connected by electrical leads to at least one remote collision sensing device.
Due to the emphasis on weight reduction for improving fuel mileage in motorized vehicles, inflator units are often formed from light weight materials, such as aluminum, that can lose strength and mechanical integrity at temperatures significantly above the normal operating temperature of the unit. Although the temperature required for the unit to lose strength and mechanical integrity is much higher than will be encountered in normal vehicle use, these temperatures are readily reached in, for example, a vehicle fire. As the operating pressure of standard pyrotechnics increases with increasing temperature, a gas generating composition at its autoignition temperature will produce an operating pressure that is too high for a pressure vessel that was designed for minimum weight. Moreover, the melting point of many non-azide gas generating compositions is low enough for the gas generating composition to be molten at the autoignition temperature of the composition, which can result in a loss of ballistic control and excessive operating pressures. Therefore, in a vehicle fire, the ignition of the gas generating composition can result in an explosion in which fragments of the inflation unit are propelled at dangerous and potentially lethal velocities.
To prevent such explosions, air bag systems have typically included inflators containing an autoignition composition that will autoignite and initiate the combustion of the main gas generating pyrotechnic charge at a temperature below that at which the shell or housing begins to soften and lose structural integrity. The number of autoignition compositions available in the prior art is limited, and includes nitrocellulose and mixtures of potassium chlorate and a sugar. However, nitrocellulose decomposes with age, so that the amount of energy released upon autoignition decreases, and may become insufficient to properly ignite the main gas generating composition charge. Moreover, prior art autoignition compositions have autoignition temperatures that are too high for some applications, e.g., non-azide auto air bag main charge generants.
Therefore, a need exists for a stable autoignition composition that is capable of igniting the gas generating composition at a temperature that is sufficiently low that the inflator unit maintains mechanical integrity at the autoignition temperature, but which is significantly higher than the temperatures reached under normal vehicle operating conditions.